Conventionally a screen printer has been used for applying such a photo-sensitive material as a photo resist and the like to the printed circuit board. The screen printer comprises a squeegee for moving on an upper side face of the screen placed on the printed circuit board and thereby pushing and applying the photo-sensitive material placed on the screen to the printed circuit board, and a doctor for uniforming the photo-sensitive material placed in uneven thickness on the upper side face of the screen after the applying operation.
In this case, when the photo-sensitive material is applied to all area of the printed circuit board having through holes for wiring and the like, the photo-sensitive material is attached to portions of the underside face of the screen which correspond to the through holes. To this end, the photo-sensitive material attached to the under face of the screen is dropped on or transferred to the printed circuit board to be subjected to the applying operation, resulting in a defective printing and a considerable reduction of a yield of products.
In order to eliminate the above-mentioned problem occurring in a conventional screen printer, there have been proposed various apparatus for cleaning the underside face of the screen and removing the photo-sensitive material left thereon.
For instance, Japanese patent publication No. 60-26709 has disclosed an apparatus in which a work piece and cleaning paper are alternatively subjected to an applying operation such that the photo-sensitive material left on the underside face of the screen is transferred to the cleaning paper to be removed from the underside face of the screen. However, there are some kinds of photo-sensitive materials which are transferred to the cleaning paper with a great difficulty, so that a sufficient transfer of the photo-sensitive material can not be obtained even in the apparatus.
In addition, Japanese laid-open utility model publication No. 60-85931 has disclosed an apparatus comprising two squeegees one of which moves on the underside face of the screen while being kept in contact therewith. However, when such a squeegee for moving on the underside face of the screen is repeatedly used for removing the photo-sensitive material, the photo-sensitive material attached to the squeegee progressively causes reduction in a photo-sensitive material-removing capacity of the squeegee and in some cases returns to the underside face of the screen.
On the other hand, as proposed in the Japanese laid-open utility model publication No. 60-85931, there is provided an apparatus comprising a sweeping roller with a sweeping cloth for removing the photo-sensitive material attached to the underside face of the screen by pressing the roller against the underside face of the screen. In the apparatus, the sweeping cloth should move in synchronized relation to a rotation of the sweeping roller in order to subsequently supply the sweeping cloth. Therefore, the apparatus has disadvantages such that a control operation for the synchronization is complicated and a trace of sweeping is left on the underside face of the screen.
Further, Japanese laid-open patent publication No. 63-178047 has disclosed an apparatus in which the photo-sensitive material attached to the underside face of the screen is swept after being molten by a solvent. In accordance with such a sweeping method by using the solvent, it is difficult to uniformly spray the solvent onto the underside face of the screen. In addition, unless the solvent is completely swept from the underside of the screen, the solvent is transferred to a printed circuit board so that the photo-sensitive material applied to the printed circuit board forms a layer having uneven thickness thereon.